OX40 ligand (OX40L) is a TNF family member; a 34 kDa type II transmembrane protein. The crystallized complex of human OX40 and OX40L is a trimeric configuration of one OX40L (trimer) and three OX40 monomers. The human extracellular domain is 42% homologous to mouse OX40L.
OX40L is not constitutively expressed but can be induced on professional APCs such as B-cells, dendritic cells (DCs) and macrophages. Other cell types such as Langerhans cells, endothelial cells, smooth muscle cells, mast cells and natural killer (NK) cells can be induced to express OX40L. T-cells can also express OX40L. The OX40L receptor, OX40, is expressed on activated T-cells (CD4 and CD8 T-cells, Th2, Th1 and Th17 cells) and CD4+Foxp3+ cells, even in the absence of activation.
The interaction between OX40 and OX40L occurs during the T-cell-DC interaction 2 or 3 days after antigen recognition. After leaving DCs, the OX40-expressing T-cell may interact with an OX40L-expressing cell other than a DC and receive an OX40 signal from this cell, which may provide essential signals for the generation of memory T-cells, the enhancement of Th2 response and the prolongation of the inflammatory responses. OX40 signals into responder T-cells render them resistant to Treg mediated suppression.
Graft versus host disease is a major cause of mortality following allogeneic bone marrow treatment. In the acute version of the disease, mature T-cells present in the bone marrow graft recognise the donor tissue as foreign in an environment of damaged tissue, which, via host APC's cause the activation and proliferation of the donor T-cells, with subsequent T-cell migration into the liver, spleen, gut, skin and lungs, causing tissue damage by the CU effector response and inflammatory cytokine/chemokine release. Onset for acute disease is usually within the first 100 days post transplantation (Hill-Ferrara, Blood May 1, 2000 vol. 95 no. 9 2754-275, Reddy-Ferrara Blood, Volume 17, Issue 4, December 2003).
Chronic GvHD usually appears 100 days post transplantation and several factors are thought to be involved, including thymic damage caused by prior acute GvHD which results in a reduced clearance of pathogenic T-cells (Zhang et al, Sep. 1, 2007 vol. 179 no. 5 3305-3314), up-regulation of TGF-β, which causes fibrosis (McCormick et al J Immuno, Nov. 15, 1999 vol. 163 no. 10 5693-5699), and a B-cell component driven by elevated B-Cell activating factor (BAFF) (Sarantopoulos et al, Clin Cancer Res Oct. 15, 2007 13; 6107) as well as auto-antibodies against platelet derived growth factor receptor (Svegliati et al, Blood Jul. 1, 2007 vol. 110 no. 1 237-241).
Clinical studies have shown that OX40 is up-regulated in both acute (Morante et al, Clinical and Experimental Immunology, 145:36-43) and chronic (Kotani et al, Blood Nov. 15, 2001 vol. 98 no. 10 3162-3164) GvHD. Administration of an antagonistic anti-OX40L enhanced survival in a lethal acute mouse model of GvHD, with a 70% survival in the treated group compared to the untreated who all died by day 43 (Tsukada et al, Blood, 1 Apr. 2000, Volume 95, Number 7) whereas treatment with an agonistic anti-OX40 Ab accelerated the disease and mortality (Blazar et al Blood May 1, 2003 vol. 101 no. 9 3741-3748). Blockade of the OX40-OX40L interaction has been shown to be efficacious in several other inflammatory disease, with anti-OX40L Ab being used to treat a mouse model of colitis (Totsuka et al., AJP-G1 Apr. 1, 2003 vol. 284 no. 4 G595-G603), and that an anti-OX40L Ab could block the development of diabetes in NOD mice (Pakala et al European Journal of Immunology Volume 34, Issue 11, pages 3039-3046, November 2004).